GEOMETRY OF CORONARY CAPILLARIES IN HYPERTHYROID AND HYPOTHYROID RAT-HEART

Coronary capillary geometry was studied in male rats treated with 3,3' ,5-triiodo-L-thyronine (T-3; Hyper), 6-N-propyl-2-thiouracil (PTU; Hyp o), or a sequence of PTU and T-3 (Hypo/Hyper). Ventricular mass and he art-to-body mass ratios revealed myocardial hypertrophy in Hyper, atro phy in Hypo, and a return of ventricular mass to control (Con) values in Hypo/Hyper rats. From cross-sectional analysis, capillary densities for Hyper and Hypo/Hyper were comparable with Con, despite increased left ventricular mass. Hypo rats demonstrated increased capillary dens ity. In Hyper and Hypo rats, tissue area surrounding individual capill aries (capillary domain) decreased, compared with Con, for capillaries close to the feeding arteriole. In Hyper and Hypo/Hyper, capillaries distal to the feeding arteriole had similar domain areas as Con; in Hy po, this area was smaller. From longitudinal analysis, capillary segme nt lengths were significantly shorter in all groups compared with. Con . Our data suggest that while hypothyroidism induced myocardial atroph y and hyperthyroidism induced myocardial hypertrophy, both thyroid sta tes stimulated capillary proliferation.